Olefins, or alkenes, are a homologous series of hydrocarbon compounds characterized by having a double bond of four shared electrons between two carbon atoms. Because of their availability, reactivity and versatility, olefins, especially the light olefins like ethylene, propylene, butenes and butadiene, continue to experience tremendous growth in demand. Ethylene, the simplest olefin member, continues to be the largest volume olefin produced.
Commercial production of ethylene is almost exclusively accomplished by pyrolysis of hydrocarbons (e.g., thermochemical decomposition of hydrocarbon feed at elevated temperatures in the absence of oxygen) in tubular reactor coils installed in externally fired heaters. Pyrolysis feedstocks include hydrocarbons such as ethane, propane or hydrocarbon liquids ranging in boiling point from that of light straight-run gasoline through gas oil through heavy oils.
The pyrolysis or thermal cracking process to produce ethylene is generally an endothermic equilibrium reaction. As an example, thermally cracking ethane to produce ethylene involves the endothermic equilibrium reaction:C2H6(g)C2H4(g)+H2(g) ΔH=+138 kJ mol−1 
Since the pyrolysis process involves endothermic equilibrium reactions, Le Chatelier's Principle suggests that high equilibrium yields of olefin products will be favored by carrying out the process at higher temperatures and lower pressures.
Because of the very high temperatures employed in carrying out pyrolysis reactions, commercial ethylene production processes invariably produce significant amounts of by-products such as acetylene, butadiene and benzene. Typically these are removed from the ethylene, e.g., so that the ethylene can be used in other conversion processes. By-product production is also problematic because they can produce high amounts of coke at pyrolysis temperatures. Coke formation is undesirable, because it can cause increased reactor pressure drop and substantial loss in heat transfer.
Steam is typically added to the pyrolysis feedstock in order to simultaneously increase ethylene yield and reduce coke accumulation. The addition of steam reduces the partial pressure of the hydrocarbon portion of the feedstock, thereby lowering reactant feed pressure to increase selectivity to ethylene. The steam also lessens coke accumulation. The amount of steam used per pound of feed in the thermal cracking process depends to some extent on the hydrocarbon feed used. Typically, steam pressures are in the range of about 30 lbs. per sq. in. (psig, 207 kPag) to about 80 psig (552 kPag). The amount of steam added is typically in the range of about 0.2 lbs of steam per pound of total hydrocarbon feed (0.09 kg/kg) to 0.7 lbs. of steam per pound of total hydrocarbon feed (0.32 kg/kg).
In a steam cracking furnace, a mixture of hydrocarbon and steam is typically subjected to temperatures of about 750° C. to 900° C., with the hydrocarbon being converted by a pyrolysis reaction to produce an effluent gas mixture typically comprising ethylene, methane, hydrogen and unconverted feed, as well as some hydrocarbons heavier than the feed. The effluent gas is cooled, e.g., by indirect contact with cooling water and/or by direct contact with circulated cooled quench oil and/or circulated cooled water. These cooling steps typically condense and at least partially remove relatively heavy hydrocarbons in the effluent stream, typically in the naphtha range and heavier.
The uncondensed cooled gas portion of the effluent stream contains ethylene and other gases having relatively close boiling points to ethylene. The effluent gas stream is typically compressed in one or more compressor stages (typically 3-5 stages) to an elevated pressure. The effluent from each stage is typically cooled against an ambient temperature medium and any condensed liquids removed before entering the subsequent compression stage. Acid gases such as H2S and CO2 are generally removed after at least one of these stages of compression, for example through the use of a caustic contacting tower or an amine scrubbing system. Once compressed, scrubbed and dried, the furnace effluent gas enters the separation section for recovering ethylene.
The separation section typically employs a number of distillation towers for the purpose of recovering ethylene from the various other compounds in the effluent gas and purifying the ethylene sufficiently for use in downstream processes, such as the manufacture of polyethylene. A number of alternatives exist for the design of the ethylene separation section. Typically, ethylene separation designs will employ at least a deethanizer tower which has the purpose of separating C2 and C3 components (that is, ethylene and ethane from propylene and propane, respectively), a demethanizer tower for separating C2 components from any components lighter than the C2 components, and a C2 splitter for the final separation of ethylene from ethane. For example, ethane-ethylene separation can be carried out at the C2 splitter at a temperature of about −30° C. to −20° C. and a pressure of about 300 psig (2068 kPag) to 350 psig (2413 kPag).
Methods of increasing the overall equilibrium production of ethylene from pyrolysis reactions are desired. In particular, ethylene pyrolysis production processes are desired, which reduce overall energy consumption. More particularly, ethylene pyrolysis production processes are desired, which reduce the compression energy needed in downstream processing to more effectively separate ethylene from various similar boiling point by-products such as ethane, acetylene and propylene.